The purpose was to study the developments of cellular immunity and serum blocking factors in a group of recipients of renal homografts. We attempted to correlate the results of these studies with various parameters in the pre-transplant period, i.e., HLA match, donor relationship and post-transplant factors such as immunosuppressive therapy. In addition, studies were begun to determine the nature of the blocking phenomenon. As reported, the 5 patients followed sequentially developed cellular immunity and serum-blocking factors in the post-transplant period. The exact time at which this occurred was somewhat variable between patients. The disappearance of cellular immunity in all cases, however, was preceded by the development of serum-blocking factors. One additional patient has been studied for a one year period during which time she has always had non-reactive lymphocytes. A 7th patient has been studied for 6 months.